This invention relates to implantable cardiac pacemakers and, more particularly, to implantable cardiac pacemakers having the capability of being programmed to operate in either a unipolar or bipolar mode of operation, and including the capability of having either unipolar or bipolar leads connected thereto.
The technology of cardiac pacemakers has developed a high level of sophistication of system performance. The current generation of cardiac pacemakers incorporates microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A background description of modern cardiac pacemaker technology is set forth in U.S. Pat. No. 4,712,555, which patent is incorporated herein by reference. It is a primary goal of programmable, multiple-mode, demand-type, cardiac pacemakers to accommodate the changing requirements of a diseased or malfunctioning heart. For example, single chamber, fixed rate pacers have been used extensively in the past to correct bradycardia, or slow heart rates. Demand pacing is employed to avoid competing rhythms in patients who have some cardiac activity. Dual chamber pacing is used to treat complete or intermittent heart block by maintaining atrio-ventricular (AV) synchrony. Various other parameters (such as rate, pulse amplitude or width, sensitivity, refractory, etc.) may also need to be altered from time to time to custom-fit the pacemaker to each patient.
Programmability has also been incorporated into pacemakers to select the type of electrodes implanted, either unipolar or bipolar. A unipolar lead is one in which stimulation occurs between the cathode tip and the pacemaker case, or anode. A bipolar lead is one in which stimulation occurs between the cathode tip, however, the anode is a ring electrode spaced approximately one inch from the cathode tip. Physicians select one lead-type over the other for a variety of reasons. A unipolar lead may be chosen due to its advantage of being physically smaller and more flexible and, therefore, easier to implant. Unipolar leads also have the advantage of being less vector sensitive for intrinsic complexes (particularly premature ventricular and atrial contractions) due to the larger dipole. On the other hand, bipolar leads provide superior noise immunity to myopotentials and electromagnetic interference. It is also known that bipolar leads eliminate pectoral muscle stimulation, however, there has also been an occasional report of diaphragmatic stimulation. Since these leads are inaccessible after implantation (except by surgical procedure), the greatest advantage of bipolar leads is its capability of being noninvasively reprogrammed to either unipolar or bipolar operation.
While electronic circuitry can be, and is, incorporated within the pacemaker itself for exercising or testing various circuit components (such as the status of battery power sources, and the effectiveness of various amplifiers, waveform shaping stages and the like), it is often more difficult to test the integrity of the leads and implanted electrodes to which the pacemaker is coupled in order to verify that such leads and electrodes can function to allow for the desired pacing operation.
At the implanting of the pacemaker and electrode system, minor damage is sometimes incurred which may affect the system's electrical insulation. This type of damage may go undetected and be without present effect on the implanted system, but the condition may manifest itself after extended time in service. When a breakdown or significant degradation of the pacemaker lead insulation occurs, it can result in a loss of sensing of intrinsic cardiac events or a loss of capture due to a lessened amount of energy reaching the cardiac tissue. Based on the underlying rhythm of the patient, this may have serious or even disastrous results. The reduced output energy reaching the heart is due to partial energy being shunted to other areas through the insulation opening.
Other types of damage can also occur to a pacemaker lead at implantation or later. A fracture in a conductor coil can affect operation by reducing the energy output to the cardiac tissue by causing a substantial increase in the lead resistance to current flow. A partial fracture will cause a reduction in output energy, while a complete fracture will result in no energy reaching the heart due to an infinite resistance (open circuit). Another type of detectable error relates to the failure of the electrode tip to be in proper contact with the heart wall.
Particular methods and apparatus for scanning the implanted leads of a pacemaker system to determine lead impedance and to detect abnormalities which may signal degradation and impending failure of pacemaker leads are the subject of U.S. patent application, Ser. No. 07/183,191, filed concurrently herewith, entitled "Lead Impedance Scanning System for Pacemakers" of Christer Ekwall, assigned to the assignee of this application. The disclosure of that application is incorporated herein by reference.
Briefly, the Ekwall application discloses a scanning system for measuring the output energy delivered to the stimulation circuit during pacing and determining the lead impedance from that measurement. The thus-determined lead impedance is compared with a moving average of the measured parameter and any deviation from that average by more than a predetermined amount is considered an anomaly. Three such anomalies in succession result in an event being counted in a first event counter for future consideration by a doctor during a patient checkup or the like. The system also monitors sensed heart signals and counts as a notable event any deviations in slope of the heart signal by more than a predetermined amount. These latter events are counted in a second counter to provide information for future reference. Thus, the Ekwall system determines the integrity of the implanted leads and electrodes by making measurements during both the pacing and sensing time intervals of the pacemaker timing cycle.
U.S. Pat. No. 4,140,131 (Dutcher et al.) discloses arrangements for detecting impedance level (either too high in the case of an open circuit, or too low in the case of a short circuit) and voltage level of the power source so that a warning may be given the patient. Basically, the lead resistance is considered as part of a voltage divider circuit and lead resistance is calculated from knowledge of impressed voltage and voltage across a known series resistor. The disclosed method is but one example of ways of measuring lead impedance which might be employed in connection with the present invention. The disclosure of U.S. Pat. No. 4,140,131 is incorporated herein by reference.
U.S. Pat. No. 4,549,548 (Wittkampf et al.) discloses a programmable pacemaker system in which the selection of lead electrodes to which pacemaker output may be connected is changed during each pacer cycle to optimize the choice of unipolar and bipolar operation for given pacemaker events. According to the patent, the selection of unipolar or bipolar mode of operation is based on a determination for monitoring the amplitude of sensed heartbeat signals to determine whether the sensing operation would be performed better in the unipolar or the bipolar mode. This is directed to a determination of heart performance vis-a-vis the leads involved so as to control the selection of unipolar or bipolar sensing.
As indicated by U.S. Pat. No. 4,606,349 (Livingston et al.), it is known in the pacemaker art to provide a pacemaker that can be programmably switched to operate in either a unipolar or bipolar mode of operation. While the Livingston et al. patent suggests that a bipolar lead always be connected to the pacemaker, which lead is then used in either a bipolar (tip-to-ring) or unipolar (tip-to-case) mode of operation, there are situations where it is desirable to use a unipolar lead. A problem thus arises when such a pacemaker has a unipolar lead connected thereto but has been programmed to operate in the bipolar mode.
Similarly, if a bipolar lead is being used in a bipolar mode of operation, and if one of the conductors breaks, the pacemaker is not able to continue operation without being reprogrammed. Unfortunately, such reprogramming can only be performed by a doctor after he has diagnosed that the conductor has broken. Then, if it is the ring conductor that has broken, the pacemaker could be reprogrammed to operate in a unipolar mode of operation using the existing implanted broken bipolar lead as a unipolar lead (tip-to-case). If it is the tip conductor that has broken, the pacemaker may, in some situations, be reprogrammed to operate in a unipolar ring-to-case mode of operation. In either event, it takes careful diagnosis and reprogramming before the pacemaker can be rendered operational. For many patients, the loss of pacemaker operation for the time period between breaking of the conductor and reprogramming of the pacemaker could present a serious health risk.
What is needed, therefore, is a pacemaker that can be programmed to operate in either a bipolar or unipolar mode of operation, that can receive either bipolar or unipolar leads, and that automatically prevents a programming configuration inconsistent with the lead used. Further, there is a need to have such a pacemaker that automatically responds to a broken conductor of a bipolar lead so as to preserve whatever pacemaker operation may be available using the remaining intact conductors of the bipolar lead. The present invention addresses these and other needs.